1. Field of Invention
The present invention relates to technologies to calculate a current position through a navigation device configured to perform route guidance by using a map.
2. Description of Related Art
As an example technology to calculate a current position through a navigation device configured to perform route guidance by using a map, the following technology is disclosed in Japanese Unexamined Patent Application Publication No. 10-332399. Namely, map-matching processing is performed so as to verify a vehicle position and/or the locus of the vehicle position against the map, where the vehicle position is calculated on the basis of a vehicle speed detected by a vehicle-speed sensor mounted on a motor vehicle and a travel-azimuth angle detected by an angular-acceleration sensor and/or a terrestrial-magnetism sensor mounted on the motor vehicle. Then, a position on a road, the position being nearest to and/or matching with the vehicle position, is calculated as the current position.
Here, in the map used by the navigation device, a road is usually defined as a set of links connected to one another through nodes, where each of the links is a line,
Further, as shown in FIG. 7A, a curve section 700 of the road is usually defined by using the above-described links and nodes. Namely, the curve section 700 is defined by a plurality of nodes 701 and a plurality of links 702. As shown in FIG. 7B, the nodes 701 are provided on the road at predetermined intervals, where each of the nodes 701 is provided at the center of the road with reference to the road width. Further, each of the links 702 connects two nodes 701 provided in the above-described manner with one another.
Technologies to estimate the curvature of a point on an actual road, the point corresponding to each of a plurality of points on the links 702 defining the road, have been proposed. An example of the above-described technologies is disclosed in Japanese Patent No. 3216461.
Hereinafter, it is assumed that the curve section 700 shown in FIG. 7A is defined by the links 702 connecting the above-described nodes 701 with one another, as shown in FIG. 7B. In that case, when the motor vehicle travels on the inner lane and/or the outer lane of the curve section 700 of the road, the locus of the vehicle position calculated on the basis of the vehicle speed and the travel-azimuth angle is different than the road shape of the curve part indicated by the links 702 on the basis of the map data. Subsequently, it is often difficult to calculate a position on at least one of the links 702, the position corresponding to the actual position of the motor vehicle, as the current position.
That is to say, when the motor vehicle travels on the outer lane of the curve part of the road, as indicated by an arrow shown in FIG. 7C, the locus of the vehicle position calculated on the basis of the vehicle speed and the travel-azimuth angle is indicated by a curve 710 shown in FIG. 7D. Thus, a difference exists between the curve 710 and a road shape 711 of the curve part, the road shape 711 being indicated by the links on the basis of the map data. Further, in that case, a difference exists between a travel distance 720 of the vehicle and a journey distance 721, and the difference accumulates and increases as the motor vehicle travels along the curve part, as shown in FIG. 7E, for example. The travel distance 720 is a distance that is calculated on the basis of the vehicle-position locus 710 and that extends from the curve entrance at each point in time. Further, the journey distance 721 is a distance along the links from the curve entrance to a position on the links, the position corresponding to the actual current position at each point in time. Here, a map-matching algorithm is used so that a position shown on the link where the previous current position was calculated is determined to be the latest current position, the position being calculated by adding to the previous position a value corresponding to a distance traveled by the motor vehicle since a point in time when the previous current position was calculated. In that case, when the above-described difference increases, it becomes difficult to calculate a current position on the map corresponding to the actual position of the motor vehicle.